Steam reheater control for turbine power plant

ABSTRACT

A moisture separator and reheater for a steam turbine power plant which uses a portion of the inlet steam in tube bundles to reheat lower pressure power plant cycle steam has a valve which controls the pressure of the reheating steam in the bundles as a proportional function of the lower pressure cycle steam.

United States Patent 1191' Woodcock Oct. 23, 1973 STEAM REHEATER CONTROLFOR 2,579,027 12/1951 Walter et al. 60/73 TURBINE POWER PLANT 2,586,5102/1952 Bryant 60/73 3,451,220 6/1969 Buscemi... 60/105 Inventor: d y J-Woodcock, Schenectady, 3,488,961 1/1970 Gerber 60/105 N.Y. [73]Assignee: General Electric Company, Primary ExaminerMartin P. SchwadronSchenectady, N.Y. Assistant ExaminerH. Burks, Sr. [22] Filed p 30 1971Attorney-William Cv Crutcher et a1.

21 A 1. N0.: 185 268 1 pp 57 ABSTRACT 52 US. Cl. 60/64 60/105 A mistureSeParmor and heater for a Steam mine [51] Int L F01, 31/16, Folk 19/10 63/04 power plant which uses a portion of the inlet steam in [58] Fieldof Search 60/105 64 73 70 tube bundles reheat Pressure POWer Plant cyclesteam has a valve which controls the pressure of [56] Reerences Citedthe reheating steam in the bundles as a proportional UNITED STATESPATENTS function of the lower pressure cycle steam. 2,540,691 2 1951Reese et a1. 60/73 4 claims, 3 Drawing Figures 1 STEAM REHEATER CONTROLFOR TURBINE POWER PLANT BACKGROUND OF THE INVENTION the supply ofheating steam to the reheater to reduce 1 stresses due to temperaturedifferences and rapid temperature changes.

Steam turbine power plants operating from an essentially saturated steamsupply, as in the case of plants supplied by boiling water orpressurized water nuclear reactors, commonly employ moisture separatingand reheating devices to remove moisture from the steam and to reheat itbetween a high-pressure turbine section and a lower pressure turbinesection. The reheaters, which may either be combined with moistureseparating elements in a single pressure vessel, or disposed in aseparate pressure vessel, commonly withdraw a portion of the inlet steamsupply to the steam turbine to serve as heating fluid for the cyclesteam which is at a lower pressure and temperature. More than one stageof reheating may be employed.

1 In the reheater, the highest temperature stage uses inlet steam as theheating fluid and the temperature on the tube-side of the reheater tubebundles does not change greatly with load. On the other hand, theshellside temperatureof the cycle steam being reheated varies over awide range during load as does .the pres- 1 sure of the steam.-Atpartial loads, significanttempera- SUMMARY OF THE INVENTION Brieflystated, the invention is practiced 'by providing a controller whichvaries the pressure of the inlet steam supplied to the reheater as afunction of the pressure of the reheated cyelesteam over selected loadranges according to the mode of operation of the reheater.

DRAWING The subject matter which is regarded as the invention isparticularly pointed out and distinctly claimed in the concludingportion of this specification. The invention, however, both as toorganization and. method of practice, together with further objectsandadvantages thereof, may best be understood by" reference to thefollowing description taken in connection with the accompanying drawingin which:'

FIG. 1 is a simplified schematic diagram of a steam turbine power plantwith reheater, illustrating the preferred controller of the invention inblock diagram form, and v FIGS. 2 and 3 are steam temperature andpressure versus steam flow diagrams under two different modes ofreheater operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 of thedrawing, a steam turbine power plant includes a high-pressure steamturbine section 1 and low-pressure turbine sections 2, with O a combinedmoisture separator and reheater vessel-3,

only one of several such units being shown for purposes of clarity.Steam entering the supply line 4 expands through the high-pressureturbine l to a wet condition,

' leaving via line 5. The cycle steam then enters moisture separator andreheater 3 at a number of inlets 6 on the bottom of the pressure shelland leaves by way of outlets 7 at the top of the pressure shell. Fromthere itv flows through line 19 to the low-pressure turbine and then tocondensers (not shown).

The interior of the moisture separator and reheater 3 contains moistureseparating elements (not shown) for removing moisture from the cyclesteam which is drained off through pipes 8. Cycle steam is then guidedvia internal baffles (not shown) over a first stage 9 of reheater tubes(shown schematically) and a second stage 10 of similar tubes. The tubebundlesof stages 9, 10 are supplied from internal headers 11 12,respectively, which connect to the tube bundles via tube sheets. Supplyand drain pipes for the headers pass through the pressure vessel walland comprise a first stage reheater inlet 13, a first stage condensatedrain 14, a second stage inlet 15 and a second stage drain 16. The firststage inlet 13 is supplied by an extraction line 17 from high-pressureturbine l. The pressure of this extraction steam varies proportionatelywith turbine load as does the cycle steam discharged through line 5 fromthe high-pressure turbine. Y

The second stage reheater inlet 15 is supplied by a branch line 18connected to the main steam supply line 4. The steam pressure andtemperature in this lineis substantially constant over the load range.

I system, shown generally at 20', is provided to control the pressure ofthe steam flowing to thesecond stage reheaterinlet 15 so that it is alinear function (proportional to) the cycle steam pressure in line 19(.or line 5). Control of the pressure in the preferred embodiment iseffected by means of a pneumatically operated steam valve 21 connectedin parallel with a motor-operated check valve 22.

The control system 20 includes a pressure transducer 23 connected tosense steam pressure'in line 19 (or, alternatively, line 5 ahead of thereheater) and to convert it to a controlling variable. A multiplyingrelay.24 serves to multiply the signal from transducer 23 by a selectedfactor K. Abypass 25 is connected to shunt relay 24 under selected modesof operation.

Reheater steam'inlet pressure downstreamof valves parator 27 withproportional plus reset characteristics, the signal in the output 28being used to adjust the opening of control valve 21 so as to reduce theerror to zero.

In order to reduce the flow requirements and size of the control valve21, the parallel-connected check valve 22 is operated to assume eitheropen or closed condition by a signal in line 29. The connections aresuch that the check valve 22 opens when the pressure drop across controlvalve 21 is less than about 50 psi, so that the valve 21 controls overonly a portion of the turbine load range.

In some installations, it may be desirable to allow control valve 21 tocontrol the inlet pressure to a pair of moisture separators andreheaters, and in this case there may be a check valve 22 for each ofthe reheaters connected in parallel with a single control valve such as21.

The components and operating medium for the control system '20 may takemany different forms, such as electronic, fluidic, hydraulic, etc. Theelements shown in the, figure are conventional pneumatic and hence areonly shown in block diagram form. By way of example, and not limitation,transducer 23 may be a Fisher Type 4157 pressure transmitter ranged 6-30psig (air) output for -175 psig (steam) input. Transducer 26 may be aFisher Type 4157 pressure transmitter ranged 6-30 psig (air) output for0-885 psig (steam) input. Multiplier 24 may be a Moore Type 68-2 2/1multiplying relay which provides a pneumatic output pressure which istwice the input pressure. Comparator 27 may conveniently be a FisherType 4l64 differential pressure controller with proportional plug resetresponse.

The moisture separator and reheater 3 shown in FIG. 1 is arranged bymeans of appropriate valving, such as valve 30, so that it can be usedeither with or without the first stage of reheating. With both the firststage 9 and the second stage 10 of reheating in service. valve 30 isopen. This is the normal mode of operation and in this case, bypass 25in the control system 20 is positioned as shown so that multiplyingrelay 24 is active. In the other mode of reheater operation, valve 30 isclosed and only the second stage reheater 10 is then supplied withsteam. In this mode, bypass 25 is positioned sov as to shunt multiplier24.

Referring now toFIGS. 2 and 3 of the drawing, FIG. 2 illustratespressure and temperature conditions obtained during normal two stageoperation of the reheater over the load range of the turbine. FIG. 3 hasimilar graph but with only the second stage reheater in operation. Theabscissa of both graphs is scaled to show the percent of steam flow tohigh-pressure turbine 1 (which is proportional to load on the turbine).Identical reference numerals are used on both graphs for purposes ofcomparison.

Variation of the controlling steam pressure in steam line 19 measured bytransducer 23 is indicated by curve 30, while the controlled steampressue in steam line 18a measured by transducer 26 is shown by curves31, 31a. Curve 31 on FIG. 2 is at twice the slope of curve 31 on FIG. 3,because the multiplying relay 24 is in operation. The controlled steampressure cannot exceed that in steam supply line 4 which is indicated bythe flat portion of curve 31a in the FIG. 2 mode. Variation oftemperature of the cycle steam on the shell-side at the inlet to thesecond stage reheater is indicated by curve 32. In the FIG. 3 mode(since the first stage reheater is not in service) curve 32 is simplythe saturation temperature corresponding to the steam pressure shown inline 30. In the FIG. 2 mode (since some first stage reheating takesplace), curve 32 is the shell-side outlet temperature from the firststage reheater.

Similarly, the temperature of the heating steam inside the tubes of thesecond stage reheater is indicated by curves 33, 33a. These correspondto saturation temperatures at the steam pressures of curves 31, 31a. Themaximum temperature difference between the second stage reheatershell-side and tube-side are shown as T. This temperature difference isa contributing factor to reheater thermal stress and is held at aminimum value by the present invention.

For purposes of comparison, the pressures and corresponding temperatureswhich would be supplied to the reheater without the control system ofthe present invention are shown by the dashed line portions 34, 35,respectively. Without the present invention, the maximum tube-sideversus shell-side temperature differential would be represented by atemperature difference T'.

OPERATION The operation of the invention in normal (or two stage)operation is as follows. From no-load up to approximately one-half load,the steam control valve 21 is controlled to cause the steam pressureentering the second stage reheater from line 18a through inlet 15 tovary in proportion to the reheater cycle steam pressure. With multiplier24 in operation, valve 21 is fully open at approximately one-half load.From approximately one-half load to full load, the temperaturedifferential is limited by the system characteristics, because thereheater shell-side inlet temperature continues to increase, while thetube-side temperature is limited to turbine inlet temperature. Thus, thecontrol system only operates from no load to one-half load.

Single stage operation takes place when the first stage reheater tubes 9are shut off using valve 30 and the bypass 25 is repositioned to shuntmultiplier 24. In this case, as, seen on FIG. 3, valve 21 serves tocontrol the inlet pressure in line 18a to the second stage reheaterinlet over the full load range in proportion to the reheat cycle steampressure. Because of the similar characteristics of the steamtemperaturewith respect to pressure in various parts of the reheater,controlling the second stage reheater steam inlet pressure linearly withreheater cycle steam pressure causes the temperature curves to generallyfollow one another. This limits the maximum temperature differential Tbetween tubeside and shell-side in the reheater.

Thus, it can be seen that the invention offers a very simple means ofcontrolling steam serving as the heat:

ing fluid in the reheater in a manner which limits its temperature overcertain load ranges in certain modes of operation. The invention makesuse of the fact that the characteristic temperature curves of heatingand heated fluids are caused to be similar. This serves to reducetemperature differentials within and around the reheater tube bundlethereby holding thermal stresses to a minimum.

The invention also offers a very simple means of controlling the heatingsteam to the reheater as load is built up on the turbine. The automaticcontrol ensures that as the turbine is loaded, so the second stagereheater heating steam pressure is gradually built up in accordance withthe turbine loading routine, thus minimizing I rates of temperaturechange in the second stage reheater, so improving reliability. Suchcontrol could only be effected manually with the most careful andtime-consuming operational procedures.

Although the invention has been shown in its preferred and simplest formas using reheat cycle steam as the controlling variable to control thepressure of the reheater supply steam, other methods of obtaining thesame linearly increasing pressure characteristic with respect to loadwill beapparent to those skilled in the art. It is desired to cover inthe appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed:

1. In a steam turbine power plant of the type having a high-pressureturbine supplied from a steam source, reheater means with tube bundlesadapted to reheat cycle steam from the high-pressure turbine using aportion of said source steam in said tube bundles as the reheatingfluid, the improvement comprising:

control means arranged to vary the pressure of said reheating fluid as afunction of the pressure of said cycle steam from said high-pressureturbine.

2. The combination according to claim 1, wherein said control meanscomprises a control valve interposed between said steam source and saidreheater means and further including comparator means controlling saidcontrol valve in response to a difference between a first signalrepresenting steam pressure at said control valve outlet and a secondsignal representing a selected multiple of cycle steam pressure.

3. The combination according to claim 2 and further including checkvalve means connected in parallel with said control valve means andarranged to open at a third signal representing a selected pressure ofcycle steam from the high-pressure turbine, whereby said control valvemeans controls over only a portion of the turbine load range.

4. The combination according to claim 2, further including selectablesignal bypass means to change the multiple of said controlling cyclesteam pressure second signal supplied to said comparaton'

1. In a steam turbine power plant of the type having a highpressureturbine supplied from a steam source, reheater means with tube bundlesadapted to reheat cycle steam from the highpressure turbine using aportion of said source steam in said tube bundles as the reheatingfluid, the improvement comprising: control means arranged to vary thepressure of said reheating fluid as a function of the pressure of saidcycle steam from said high-pressure turbine.
 2. The combinationaccording to claim 1, wherein said control means comprises a controlvalve interposed between said steam source and said reheater means andfurther including comparator means controlling said control valve inresponse to a difference between a first signal representing steampressure at said control valve outlet and a second signal representing aselected multiple of cycle steam pressure.
 3. The combination accordinGto claim 2 and further including check valve means connected in parallelwith said control valve means and arranged to open at a third signalrepresenting a selected pressure of cycle steam from the high-pressureturbine, whereby said control valve means controls over only a portionof the turbine load range.
 4. The combination according to claim 2,further including selectable signal bypass means to change the multipleof said controlling cycle steam pressure second signal supplied to saidcomparator.